Composite materials are used as constructional materials in a variety of technical fields such as in aerospace applications, aircrafts, wind power generation, automobiles, and marine applications. Such composite materials are more lightweight than metal materials and are therefore expected to be demanded in automobile industry or the like mainly for the purpose of, for example, improvement of fuel efficiency.
For the use in automobile industry or the like, materials using thermoplastic resins have been investigated from the viewpoint of productivity and recyclability. However, thermoplastic resins have high viscosity, and thus mixing a thermoplastic resin with a reinforcing material such as fibers involves the problems of formation of voids at the interface between the resin and the reinforcing material and uneven distribution of the reinforcing material, because of which the resulting mechanical strength is low. Further, these problems substantially preclude dense filling with the reinforcing material.
In an effort to improve the degree of impregnation of a reinforcing material such as reinforcing fibers, Patent Literature 1 discloses a method in which the reinforcing material is impregnated with a low-viscosity monomer such as ε-caprolactam or cyclic butylene terephthalate and then the monomer is converted to a polymer by ring-opening polymerization. Such polymerization generates substantially no gases and allows production of a composite material free of voids. However, there were the following problems: the polymerization from a monomer cannot yield a sufficiently high molecular weight to achieve a high strength; and the type of the resin is limited.
According to Patent Literature 2, a chain extender is added to a reactive prepolymer of low viscosity, and further polymerization is performed after impregnation. This requires the step of mixing the prepolymer and the chain extender before fiber impregnation and is disadvantageous in terms of working efficiency.
Patent Literature 3 describes a method for obtaining a composite material with high mechanical strength by impregnation of reinforcing fibers with a polyamide oligomer, followed by pultrusion molding. With this method, small molecules formed during polymerization of the prepolymer remain, and the remaining small molecules may cause polymerization to proceed insufficiently or voids to be formed, apprehensively leading to poor properties of the resulting composite material.